Bytownite: The plagioclase feldspar mineral Bytownite info & pictures

About

Bytownite is a rarer form of feldspar, more commonly seen as a faceted gemstone then as a collectors mineral. It is usually translucent without a crystal form. Some other forms of feldspar, especially the transparent yellow form of Orthoclase, are faceted and incorrectly labeled as Bytownite.

Bytownite belongs to the Plagioclase Feldspar group, an isomorphous solid solution series. Albite is one member, containing sodium and no calcium. The other end member, Anorthite, contains calcium and no sodium. Bytownite is an intermediary member of this series. Bytownite is considered by some authorities as a variety of Anorthite rather then a separate mineral. The acclaimed Dana's System of Mineralogy lists Bytownite as an individual mineral, whereas the IMA does not recognize it as individual mineral species, but rather a sodium-rich variety of Anorthite.

Identification

Chemical Formula: (Ca,Na)Al_1-2Si_3-2O₈
Composition: Sodium calcium aluminum silicate. The ratio of sodium to calcium is about 1:9. The amount of aluminum atoms are between 1 and 2, and the amount of silicon atoms are between 3 and 2.

In the Plagioclase Feldspar series, Bytownite contains between 10 and 30 percent Albite (Ab), and between 70 and 90 percent Anorthite (An).
Color: Colorless, white, cream, pale yellow, yellow-brown, brown, gray
Streak: White
Hardness: 6–6.5
Crystal System: Triclinic
Luster: Vitreous. Pearly on cleavage surfaces.
Cleavage: 2,1 - basal ; 2,1 - prismatic ; 3,1 - pinacoidal. The cleavage angle is about 90º.
Fracture: Conchoidal to uneven
Tenacity: Brittle
Transparency: Transparent to translucent
Specific Gravity: 2.72–2.74
Group: Silicates; Tectosilicates; Feldspar Group
Rock Type: Igneous, Metamorphic

Rankings

Popularity (1–4): 2
Prevalence (1–3): 3
Demand (1–3): 2

Crystal Forms & Aggregates

Rarely occurs in crystals, which are tabular and usually twinned. Most often massive, grainy, and as compact crystal groupings. Also as blocky crystal fragments.

Striking Features

Crystal habits, localities, and cleavage

Environment

Igneous dikes and in metamorphic rock.

Uses

The yellow and brown transparent forms of Bytownite are faceted as a rare feldspar gemstone.

Noteworthy Localities

Bytownite is a rare mineral, and occurs in many scattered localities usually lacking specimens of interest. It is named after the locality of Bytown, which was the old name for Ottawa (the capitol of Canada) where this mineral was first described. Only a handful of old specimens are noted from this locality. Bytownite occurrences include the Dorado Mine, Casas Grandes, Chihuahua, Mexico; Santiago Papasquiaro, Durango, Mexico; Plush, Lake Co., Oregon; and the vicinity of Pueblo Creek, Gila National Forest, Catron Co., New Mexico.

Common Mineral Associations

Quartz, Muscovite, Biotite, Hornblende

Distinguishing Similar Minerals

Several minerals can be confused with Bytownite, but the localities and cleavage can usually distinguish it from all other minerals.

Bytownite

Abouti

Crystal Forms & Aggregatesi

Striking Featuresi

Environmenti

Usesi

Noteworthy Localitiesi

Common Mineral Associationsi

Distinguishing Similar Mineralsi

Photos